With the recent high consciousness of environmental problems such as global warming, there is an increasing demand, in particular, for solar power generation systems in view of cleanness and pollution-free nature thereof. The solar cells serve as a central constituent part of the solar power generation systems capable of directly converting a solar energy into electricity. In general, the solar cells have a structure in which a plurality of solar cell elements (photovoltaic cells) are arranged in series or in parallel with each other by wiring, and these cells are protected by various packaging methods to provide a unit thereof. The unit incorporated in the package is called a “solar cell module”. The solar cell module is generally constructed from a transparent substrate (glass/transparent solar cell sheet; front sheet) as an upper protective material covering a surface of the cells which is irradiated with sunlight, an encapsulant material (encapsulant resin layer) made of a thermoplastic resin (such as, e.g., ethylene-vinyl acetate copolymers) which is filled in a clearance between the cells, and a back surface sealing sheet (back sheet) as a lower protective material for protecting a back surface of the cells.
Since the solar cell module is mainly used outdoor, materials and structures used therein are required to exhibit various excellent properties. The properties mainly required for the encapsulant material include a flexibility or an impact resistance for protecting the solar cell elements, a heat resistance capable of withstanding heat generated from the solar cell module, and a transparency (such as total light transmission) for efficiently transmitting sunlight to the solar cell elements as well as a durability, a dimensional stability, a flame retardancy, a water vapor-barrier property, etc.
At present, as the encapsulant material for the solar cell elements in the solar cell module, ethylene-vinyl acetate copolymers (hereinafter occasionally referred to merely as “EVA”) have been extensively used as a raw material thereof from the viewpoints of a good flexibility, a high transparency or the like (for example, refer to Patent Document 1). In addition, the EVA is crosslinked using a crosslinking agent such as an organic peroxide mainly for the purpose of imparting a heat resistance to EVA. Therefore, there is usually employed such a step of encapsulating the solar cell elements using an EVA sheet previously prepared by mixing the EVA with the crosslinking agent (organic peroxide) or a crosslinking assistant. Upon production of the EVA sheet, it is required that the sheet forming is carried out at a low temperature (usually in the range of from about 80 to about 100° C.) so as not to decompose the organic peroxide, which therefore makes it difficult to increase an extrusion molding rate used upon forming the sheet. Also, the encapsulating of the solar cell elements must be performed through two-stage steps including a step in which air relief or pre-bonding is carried out over a period of from several minutes to ten and several minutes in a laminator, and a step in which substantial bonding (crosslinking) is carried out over a period of from about ten and several minutes to about 60 minutes at an elevated temperature capable of decomposing the organic peroxide (usually in the range of from about 130 to about 150° C.) in an oven. For this reason, the production process of the solar cell module requires larger man-hour and prolonged time, thereby causing problems such as increase in production costs.
In addition, the encapsulant material for the solar cell elements which is obtained by using the EVA sheet has such a problem that a solar cell circuit is corroded or tends to be corroded with acetic acid generated due to hydrolysis of EVA, etc., when used for a long period of time. Further, there also tends to occur such a problem that peeling of the respective materials is caused owing to the crosslinking agent, the crosslinking assistant or acetic acid generated therefrom at an interface between the EVA sheet and the solar cell elements, an interface between the EVA sheet and the front sheet or an interface between the EVA sheet and the back sheet.
To solve these conventional problems, for example, Patent Document 2 discloses an encapsulant material for solar cells which is composed of a resin composition containing a non-crystalline α-olefin polymer and a crystalline α-olefin polymer, in which the solar cells are encapsulated without using the EVA sheet, and therefore the encapsulating procedure is simplified by omitting a crosslinking step. More specifically, as the encapsulant material of Patent Document 2, the resin composition composed of a polymer containing propylene as a main component is used.
Also, Patent Document 3 discloses an encapsulant material for solar cells which is in the form of a polymer blend or a polymer alloy composed of at least one polyolefin-based copolymer and at least one crystalline polyolefin. More specifically, as the encapsulant material, there are described a polymer blend of a lower-melting point EVA and a higher-melting point EVA (refer to Example 1 of Patent Document 3), a polymer blend of an ethylene-methacrylic acid copolymer and an ordinary crystalline polyethylene (refer to Example 2 of Patent Document 3), and a polymer blend of an ethylene-methyl acrylate copolymer and an ordinary crystalline polypropylene (refer to Example 3 of Patent Document 3).    Patent Document 1: JP-A 58-60579    Patent Document 2: JP-A 2006-210905    Patent Document 3: JP-A 2001-332750